1. Field of the Invention
The present invention relates to the technical field of LCD devices and, more particularly, to a gate pulse modulation waveform-shaping circuit.
2. Description of Related Art
Currently, the liquid crystal display (LCD) is the most common flat display device, which has the advantages of compactness, low power consumption, and low radiation. The operating principle of LCD is to regulate the voltage difference between two ends of a liquid crystal layer to change the arrangement of liquid crystal molecules in the liquid crystal layer thereby changing the transmittance of the liquid crystal layer, and to further make use of a light source provided by a backlight module to display an image.
The LCD device includes a plurality of pixel units, a source driver, and a gate driver. The source driver provides a plurality of data signals to the plurality of pixel units. The gate driver generates a plurality of gate signals to activate the corresponding pixel units and thus control a write operation of the data signals. The gate driver is connected to the scan lines of an LCD panel, and there are parasitic capacitance and resistance on the scan lines. Due to the resistance-capacitance (RC) load effect, signal on the scan line which is distant from the gate driver may present a dragging situation, so that the waveform of the signal on the scan line which is close to the gate driver is quite different from that distant from the gate driver, resulting in that the brightness at left and right sides of a frame is different.
To overcome this problem, the prior art makes use of a gate pulse modulation (GPM) to lengthen the falling edge of the waveform so as to reduce the waveform difference of the signal on the scan line close to and distant from the gate driver. FIG. 1 is a schematic diagram of a typical gate pulse modulation unit 120. As shown in FIG. 1, the GPM unit 120 is arranged in a pulse width modulation integrated circuit (PWM IC) 110. The GPM unit 120 turns a PMOS transistor 130 on and turns a PMOS transistor 140 off at a high potential VGH of a control signal GVOFF, and turns the PMOS transistor 130 off and turns the PMOS transistor 140 off at a low potential VGL of the control signal GVOFF. Due to the RC effect, the falling edge of the signal VGHM is lengthened to thus reduce the waveform difference of the signal on the sane line close to and distant from the gate driver. Accordingly, the brightness at two ends of a frame becomes similar.
As shown in FIG. 1, the capacitance C is an equivalent capacitance on the scan line, but not a physical capacitor. The function of the typical GPM unit 120 is to control the slope through the RC (resistance and capacitance) discharge. However, because different LCD panels require different slopes, the value of the resistance R has to be adjusted to satisfy the actual requirement, which may cause a material preparation problem on production as different types of LCD panels require different resistance values. Another approach is to use a variable resistor to perform the resistance adjustment in the PWM IC 110. However, such a way to implement the resistor in the IC may overheat the IC, and even burn down the IC. Therefore, it is desirable to provide an improved gate pulse modulation waveform-shaping circuit to mitigate and/or obviate the aforementioned problems.